<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title></title>
	</head>
	<body>
		<script type="text/javascript">
//			alert(event) //undefined
			window.onload = function  (evt) {
//				alert(event)   //object event
                var ev = evt || event
			}
			//必须要在事件发生时才有值，否则就是undefined
			
			//兼容性1.
		</script>
	</body>
</html>
